There are several types of fuel cells currently available, all with specific advantages and disadvantages. Currently, work has been intensifying in the area of methanol reformers, devices that can convert methanol and water into hydrogen and carbon dioxide. The hydrogen from such a device can be used to run a fuel cell. Typically, these reformers operate at 200-300° C., and produce several tenths of a percent carbon monoxide in their effluent stream. Proton exchange membrane (PEM) fuel cells typically operate at <85° C. At these temperatures, more than 100 ppm carbon monoxide in the fuel stream is typically poisonous to the anode catalyst of a fuel cell. In order to alleviate this condition, a preferential oxidizer, or PROX, is used to selectively oxidize carbon monoxide in the fuel stream to carbon dioxide, while leaving most of the hydrogen unreacted, before it reaches the fuel cell. The preferential oxidizer lowers the carbon monoxide levels to less than 100 ppm, yet this is still enough to poison the anode of the low temperature PEM fuel cell.
A fuel cell that operates at 80° C., while intrinsically a fairly efficient device, nonetheless liberates about 50% of the energy in the fuel stream as heat. At such temperatures, this waste heat is of low quality and cannot be used to drive the reforming reaction, which is endothermic. Typical proton exchange membranes, such as Nafion, also require aggressive humidification for optimal ionic conductivity and peak performance. Supplying the fuel cell with both fuel and oxidant gas streams at near saturated levels increases system complexity. Also; a fuel cell is a device that creates water as product, while typically necessitating a tight operating window where conditions must be delicately balanced between saturation for optimum performance, while avoiding condensing conditions, which chokes off gas access to the electrodes and degrades performance.
Thus, a nominal fuel cell is preferably ideally suited, both thermally and chemically, for operation in conjunction with a methanol or other hydrocarbon fuel reformer. Phosphoric acid fuel cells (PAFCs) are well suited for these conditions, as they can operate at higher temperatures.